In recent years, aiming to make functional devices such as a semiconductor device, a MEMS device, and a display device, lightweight, compact and thin, and flexible, techniques for forming these devices on a polymer film have actively been developed. For example, as a material for a base of electronic parts such as information and communication equipments (broadcast equipment, mobile radio equipment, portable communication equipment, etc.), radars, and high speed information processing equipments, ceramics having heat resistance and being capable of dealing with high frequency in a signal band (reaching GHz band) for information and communication equipments have conventionally been used; however, ceramics are not flexible, hard to be made thin, and therefore there is a disadvantage that the fields for which ceramics are applicable are limited.
In the case of forming a functional device such as a semiconductor device, a MEMS device, or a display device on a polymer film surface, it is supposed to be ideal that the processing utilizing flexibility, a characteristic of a polymer film, so-called roll-to-roll processing is employed for the processing. However, in semiconductor industries, MEMS industries, and display industries, processing techniques for rigid flat basal plates such as a wafer base and a glass substrate base have been established so far. Therefore, as a realistic selection, it is supposed to be possible that a polymer film is stuck to a rigid substrate made of an inorganic substance, for example, a glass plate, a ceramic plate, a silicon wafer, or a metal plate; a desired device is formed; and thereafter, the polymer film is peeled from the substrate; and thereby a functional device formed on the polymer film is obtained by utilizing existing infrastructure.
Conventionally, it has been widely carried out that a polymer film is stuck to a substrate made of an inorganic substance by using a pressure-sensitive adhesive or an adhesive (Patent Document 1). However, in the case of forming a desired functional device on a laminate formed by sticking a polymer film and a substrate made of an inorganic substance, the laminate is required to have surface smoothness, dimensional stability, cleanness, durability to processing temperature, resistance to chemical solution to be used for fine processing, etc., high enough to carry out the formation of the functional device. Particularly, in formation of a functional device such as polysilicon and oxide semiconductors, processing in a temperature range of about 200 to 500° C. is required. For example, in fabrication of a low temperature polysilicon thin film transistor, heating treatment at 450° C. for about 2 hours for dehydrogenation is may be required and in formation of a hydrogenated amorphous silicon thin film, it may be possible that the film is exposed to a temperature of about 200° C. to 300° C., In the case where the functional device formation temperature is as high as described above, not only the polymer film is required to have high heat resistance but also the bonding surface between the polymer film and the substrate (that is, an adhesive or a pressure-sensitive adhesive for bonding) needs to withstand the processing temperature. However, since conventional adhesives and pressure-sensitive adhesives for bonding do not have sufficient heat resistance, they are not currently applicable in the case where the functional device formation temperature is high.
Further, among semiconductor thin films, in the case of forming a Si thin film with a very low coefficient of thermal expansion as low as about 3 ppm/° C. on a polymer film, if the difference between the coefficient of thermal expansion of the film and that of the thin film is high, the stress is accumulated in the thin film and it results in problems of causing deterioration in performance, warping and peeling. Particularly, in the case where high temperature is applied during the thin film formation process, the stress attributed to the difference between the coefficient of thermal expansion of the film and that of the thin film becomes significant during temperature change.
As the polymer film to be stuck to the substrate made of an inorganic substance, films with a low melting point is not suitable in terms of heat resistance, and polymer films made of polyethylene naphthalate, polyethylene terephthalate, polyimide, and polytetrafluoroethylene, glass fiber-reinforced epoxy, and the like are used. Particularly, a film made of a polyimide is provided with advantages such that the film is excellent in heat resistance, is also tough, and therefore is capable of being formed into a thin film. However, a polyimide film generally has a high coefficient of thermal expansion, shows considerable dimensional change owing to temperature change, and thus has a problem that it is difficult to use the film for producing a circuit having fine wiring, etc., and is limited in the fields of using the film. Accordingly, a device using a polyimide film with sufficient physical properties as a basal plate provided with heat resistance, high mechanical physical properties, and flexibility has not been obtained yet.
As a polyimide film with high tensile elasticity, a polyimide-benzoxazole film made of a polyimide having a benzoxazole ring in the main chain has been proposed (Patent Document 2). Further, a printed circuit board having this polyimide-benzoxazole film as a dielectric layer has been also proposed (Patent Document 3 and Patent Document 4). However, the polyimide-benzoxazole film made of a polyimide having a benzoxazole ring in the main chain is made to have improved tensile strength at break and tensile elasticity, and further a coefficient of thermal expansion in a satisfactory range; however, contrarily to the excellent mechanical properties, it becomes difficult to handle the film as the film is made thinner, and the film has a problem that the mechanical and dynamic properties are insufficient, etc.
It has also been tried to form other structurally-reinforced materials by forming an adhesive layer such as a thermoplastic resin on the polyimide film. However, although providing satisfactory improvement in terms of rigidity, the trial tends to sacrifice the valuable heat resistance of the polyimide film because of the low heat resistance of the thermoplastic resin or the like as an adhesive layer. Moreover, a thermoplastic resin generally has a high coefficient of thermal expansion and thinning of this layer is limited so that the thermoplastic resin tends to cause an adverse effect on the dimensional stability at the time of heating.
On the other hand, as a flexible display apparatus using a resin basal plate, there is disclosed fabrication of a flexible display apparatus using a resin basal plate, the fabrication including the steps of: forming the resin basal plate on a fixed basal plate with an amorphous silicon film as a release layer interposed therebetween; forming at least a TFT device on the resin basal plate; and peeling the resin basal plate from the fixed basal plate through the amorphous silicon film by irradiating the amorphous silicon film with laser beams (Patent Document 5). However, at the time of peeling, laser irradiation or etching means needs to be used to the adhesive layer, and the steps are thus complicated, resulting in high cost.
In addition, it is known that adhesion of polymer films is performed by UV irradiation, and disclosed that use of a coupling agent at this time is effective (Patent Document 6). However, this technique persistently relates to adhesion of polymer films, but do not relate to control of the adhesion peel force of the coupling agent itself by UV irradiation.